Light-transmitting alumina ceramics, both the clear synthetic sapphire (single crystal) and the translucent polycrystalline material made in accordance with U.S. Pat. No. 3,026,210--Coble, are suitable for use as the arc tube in alkali metal-containing arc lamps. The polycrystalline material is cheaper and is almost universally used in high intensity sodium vapor lamps for outdoor lighting as in streets and public places. The method of preparing this ceramic involves sintering a pressed compact of finely divided alumina powder containing a small but effective amount of finely divided magnesia not exceeding 0.5 weight percent, at elevated temperatures in the range of 1700.degree. to 1950.degree. C., either in vacuum or in hydrogen. The sintering is continued long enough to produce a relatively uniform large grain size alumina structure which contains little or essentially no secondary magnesia alumina phase at the grain boundaries.
The lowest cost method of preparing alumina ceramic tubing for use as lamp bodies involves extruding a stiff paste of alumina powder in tubular form in lengths of 1 meter or more, sintering it, and then cutting the polycrystalline tubing into appropriate lengths. By way of example, the basic ingredients consisting of 99.9% pure Al.sub.2 O.sub.3 with not over 0.5 wt% MgO are thoroughly mixed in the proper proportions, blended with a liquid organic binder suspension containing a stearate lubricant, and then compacted in an extruder from which it is extruded as tubing under a pressure in the range of 20 to 35 tons. The "green" density of the tubing thus prepared attains from 30 to 35% of the theoretical density of a single crystal of alumina. The raw tubing is next presintered in air for several hours at a temperature from 950.degree. to 1000.degree. C. The presintered material is then finally sintered at approximately 1900.degree. C. for about 4 hours in an electric furnace provided with a hydrogen atmosphere. The resulting product consists of translucent polycrystalline alumina tubing, a typical size being 7.2 mm i.d., 0.5 mm wall thickness, and about 1 meter in length.
In order to utilize the tubing for lamp bodies, it must be cut into appropriate lengths, for instance about 9 cm. long for a 400 watt high pressure sodium vapor lamp. Up to now, this has been done using wet diamond saws which are relatively expensive and have a limited life. Also the sawing generates appreciable detritus, as a result of which the tubes have to be washed and dried after cutting. A considerable amount of handling is required for these operations and chipping or cracking of the tubes is common.